My maternal grandfather was a Mennonite, and he and his siblings all went native, leaving their Kansas farm. An interesting story in the Wall Street Journal on a Pennsylvania Mennonite family that took it in the other direction. The family doesn't have medical insurance, but as nine of his 11(!) children suffer from serious diseases, he racks up some big bills in emergency treatments and surgeries. One son was diagnosed with a disorder called 'maple syrup urine disease', in which patients can't break down certain amino acids, and sounds rather gross. Other children had Hirschsprung disease, an inherited disorder that causes an obstruction in the colon. It is fatal without immediate surgery.
The Amish community probably doesn't have a huge amount of genetic diversity as it is, but this guy married his first cousin. You don't have to be a rocket surgeon to figure this one out.